Process for preparing pyridylmethyl isothiocyanates

ABSTRACT

The present invention relates to a novel process for preparing pyridylmethyl isothiocyanates of the formula (I) ##STR1## in which R 1  represents halogen or C 1  -C 4  -alkyl, characterized in that in a first step, amines of the formula (II) ##STR2## in which R 1  is as defined above, are reacted with xanthogenates of the formula (III) 
     
         R.sup.2 O--CS--S.sup.⊖ M.sup.⊕                 (III) 
    
     in which 
     R 2  represents C 1  -C 4  -alkyl, preferably methyl or ethyl and 
     M represents an alkali metal, preferably sodium and potassium or represents ammonium, 
     if appropriate in the presence of a diluent, and in a second step, the resulting novel dithiocarbamates of the formula (IV) ##STR3## in which R 1  and M are each as defined above, are oxidized, if appropriate in the presence of a diluent, to give the isothiocyanates of the formula (I).

This is a divisional application of application Ser. No. 09/116,267, onJul. 16, 1998, now U.S. Pat. No. 5,936,093.

The present invention relates to a novel process for preparingpyridylmethyl isothiocyanates and to novel intermediates.

It is known that pyridylmethyl isothiocyanates are obtained whenpyridylmethylamines of the formula (A) are reacted with carbon disulfideand, for example, ethyl chloroformate in the presence of a base, suchas, for example, sodium hydroxide, according to the following equation:##STR4## Py=optionally substituted pyridyl, such as, for example,2-chloro-5-pyridyl (cf. EP-A 0 302 389).

However, this process has the disadvantage that carbon disulfide isemployed as reaction partner. Furthermore, carbon oxysulfide is obtainedas byproduct.

It has now been found that pyridylmethyl isothiocyanates of the formula(I) ##STR5## in which R¹ represents halogen or C₁ -C₄ -alkyl areobtained in good yields and high purity when in a first step, amines ofthe formula (II) ##STR6## in which R¹ is as defined above, are reactedwith xanthogenates of the formula (III)

    R.sup.2 O--CS--S.sup.⊖ M.sup.⊕                 (III)

in which

R² represents C₁ -C₄ -alkyl, preferably methyl or ethyl and

M represents an alkali metal, preferably sodium and potassium orrepresents ammonium,

if appropriate in the presence of a diluent, and in a second step, theresulting dithiocarbamates of the formula (IV) ##STR7## in which R¹ andM are each as defined above, are oxidized, if appropriate in thepresence of a diluent, to give the isothiocyanates of the formula (I).

The formula (I) provides a general definition of the pyridylmethylisothiocyanates preparable according to the invention. In this formula,R¹ preferably represents chlorine or methyl.

Surprisingly, the pyridylmethyl isothiocyanates of the formula (I) canbe obtained in good yields and in high purity by the process accordingto the invention, although the sulfur-containing radical MSH in thexanthogenates of the formula (III) is the better leaving group ascompared to the alcohol radical R² OH and the following course ofreaction could therefore have been expected: ##STR8##

The reaction according to the invention has the advantage that carbondisulfide is not required as reaction partner. Moreover, the practice ofthe reaction is simple (no exothermic reaction) and the reactionproceeds without side reactions.

Using, for example, 5-aminomethyl-2-chloropyridine and potassiumethylxanthogenate as starting materials and aqueous NaOCl solution asoxidizing agent, the course of the reaction of the process according tothe invention can be illustrated by the following equation: ##STR9##

The amines of the formula (II) to be used as starting materials in thefirst step of the process according to the invention are known (cf., forexample, EP-A 0 391 205 or U.S. Pat. No. 4,499,097) and/or can beobtained in a generally known manner.

The xanthogenates of the formula (III) furthermore to be used asstarting materials in the first step of the process according to theinvention are generally known compounds.

The formula (IV) provides a general definition of the dithiocarbamatesobtainable in the first step of the process according to the invention.In this formula, R¹ preferably represents chlorine or methyl and Mpreferably represents sodium, potassium or ammonium.

The dithiocarbamates of the formula (IV) are novel and also form part ofthe subject matter of the present application.

Preferred oxidizing agents for the second step of the process accordingto the invention are: sodium hypochlorite or potassium hypochloritesolution and copper(II) compounds, such as copper sulfate.

Suitable diluents for the first step of the process according to theinvention are customary organic solvents. These preferably includehydrocarbons, such as toluene or xylene; ethers such as methyltert-butyl ether, methyl tert-amyl ether, 1,2-dimethoxyethane,1,2-diethoxyethane, tetrahydrofuran or dioxane; alcohols, such asmethanol, ethanol, n-propanol, i-propanol, n-, i-, s- or t-butanol;nitrites, such as acetonitrile, propionitrile or butyronitrile; andamides, such as dimethylformamide.

When carrying out the first step of the process according to theinvention, the reaction temperatures can be varied within a relativelywide range. In general, the reaction is carried out at temperaturesbetween 30° C. and 150° C., preferably at temperatures between 50° C.and 120° C.

When carrying out the first step of the process according to theinvention, preference is given to employing equimolar amounts. However,it is also possible to employ a relatively large excess of the cheaperxanthogenate of the formula (III), preferably 100%.

The reaction is carried out and the novel dithiocarbamates of theformula (IV) are worked up and isolated in a generally customary manner(cf. also the Preparation Example).

Preferred diluents for the oxidation according to the second step of theprocess according to the invention are water and a second inert,sparingly water-miscible solvent, such as, for example, hydrocarbons,chlorinated hydrocarbons, ethers, nitrites, ketones or amides.

When carrying out the second step of the process according to theinvention, the reaction temperatures can be varied within a relativelywide range. In general, the reaction is carried out at temperaturesbetween -30° C. and 50° C., preferably at temperatures between -10° C.and 10° C.

When carrying out the second step of the process according to theinvention, generally 4 to 5 mol, preferably 4 to 4.5 mol, of oxidizingagent are employed per mole of dithiocarbamate.

The reaction is carried out and the pyridylmethyl isothiocyanates of theformula (I) are worked up and isolated in a generally customary manner(cf. also the Preparation Example).

The pyridylmethyl isothiocyanates of the formula (I) to be prepared bythe process according to the invention can be used as intermediates forpreparing biologically active compounds, for example insecticides (cf.,for example, EP-A 0 302 389).

PREPARATION EXAMPLES Example 1 ##STR10## 51.3 g (0.2 mol) of potassium2-chloro-5-pyridylmethyldithiocarbamate (cf. first step) are dissolvedin 200 ml of water and admixed with 200 ml of methylene chloride.

At 0° C., 460 ml of 13% strength aqueous NaOCl solution are addeddropwise in such a manner that the temperature does not exceed 5° C. Themixture is stirred at 0-5° C. for 30 minutes, the organic phase isseparated off, the aqueous phase is extracted three times with methylenechloride and the combined organic phases are dried with sodium sulfate.

The solvent is distilled off, giving 31.3 g (82% of theory) of2-chloro-5-pyridylmethyl isothiocyanate which crystallizes in the fridge(melting point: 22° C.) ##STR11## 2.85 g (20 mmol) of2-chloro-5-aminomethyl-pyridine and 3.2 g (20 mmol) of potassiumethylxanthogenate in 30 ml of ethanol are stirred under refluxovernight.

The mixture is allowed to cool and filtered off: This gives 4.4 g (86%of theory) of potassium 2-chloro-5-pyridylmethyldithiocarbamate ofmelting point 252° C. (decomp.).

What is claimed is:
 1. A dithiocarbamate of the formula (IV): ##STR12##in which R¹ represents halogen; andM represents an alkali metal selectedfrom the group consisting of sodium and potassium, or representsammonium.
 2. The dithiocarbamate according to claim 1, wherein Mrepresents an alkai metal selected from the group consisting of sodiumand potassium.
 3. The dithiocarbamate according to claim 1, wherein Mrepresents ammonium.